The background of this application is that, generally described, three different methods are used today for cylinder lubrication.
A first method comprises conventional cylinder lubrication.
For this is used a system with mechanical lubricating apparatuses which are driven directly via the chain drive of the engine. Synchronous operation of lubricating apparatus and engine is hereby achieved. Such a system typically consists of mechanical lubricating apparatus with a piston pump and associated check valves. At the outlet of the lubricating apparatus, a check valve is provided which through a lubricating oil tube is coupled to an injecting unit (injector/check valve). In this type of system, the oil is supplied to the cylinder immediately before the uppermost piston ring of the piston passes the injection unit. Lubricating oil is typically supplied to the cylinder by each engine stroke.
In these conventional cylinder lubricating apparatuses, mainly for large two-stroke diesel engines, two or more central lubricating apparatuses are used, each providing lubrication at points in a single or a plurality of cylinders, i.e. by feeding portions of oil under pressure through respective connecting lines to the various points to be lubricated at relevant time intervals. These relevant intervals may typically be when the piston rings are provided opposite the relevant point of lubrication during the compression stroke when the piston is moving upwards.
A second method for cylinder lubrication appears on more recent engines and is described as high-speed cylinder lubrication.
Hydraulically powered lubricating apparatuses are used for this purpose where the mechanical chain drive is substituted by a hydraulic system which is timed via timing sensors mounted directly on the flywheel of the marine engine. By this kind of cylinder lubrication, a piston pump is typically used as well. In this kind of system, the lubricating oil is fed into the cylinder simultaneously with the passage of the piston such that largely all the lubricating oil is supplied directly onto the piston, typically between the uppermost and the lowermost piston ring. When the lubricating oil is supplied between the piston rings, it is expected that they retain the lubricating oil better and that the piston subsequently distributes the oil along the travel path of the piston. There are also systems as e.g. disclosed in WO 2008/009291 where hydraulically powered apparatuses are used, where both the injected amount and the timing for delivery of the latter may be adjusted.
The lubricating oil is supplied intermittently such that the amount is adjusted on the basis of the frequency of activation of the piston pump as the stroke of the piston pump is constant. The lubricating oil is supplied by these systems via an injection unit that includes a traditional check valve, injector or an atomising valve. Examples of this technique are known from e.g. DK 173 512 or DE 101 49 125.
There are variants of this high-speed lubrication. Thus is provided a system where the piston pump principle is not used. Instead, the injected amount of lubricating oil is controlled by controlling the opening and closing time. An example of this technique is known from e.g. EP 1 426 571.
The injection may occur by the passage of the piston in upward or downward direction. If this occurs during the downward movement, the oil is distributed on the cylinder face from the point to the lubricated and down in the cylinder lining. However, it is preferred to perform the injection during the upward passage of the piston against the hot end of the cylinder where the need for lubrication is the greatest.
The traditional way by which oil is distributed across the cylinder surface is by establishing two inclining grooves or slots at each point to be lubricated on the cylinder surface, where both grooves or slots initiate from the lubrication point and are directed away from the top of the cylinder. When a piston ring passes such a slot, a drop of pressure occurs in the slot across the piston ring which presses the oil away from the lubrication point. These and other methods, however, have appeared insufficient in that in practice there is observed a substantial variation in the wear occurring along the periphery of the cylinder.
The development towards still greater utilisation of the engines have resulted in an increased mechanical and thermal load on cylinder linings and piston rings, which is traditionally enabled by an increase in the dosing of lubricating oil. However, it has appeared that if the dosing is increased above a certain limit which is not unambiguously defined, the speed of the oil when injected into the cylinder with the mentioned traditional lubrication is so high that instead of remaining on the cylinder face, it forms a jet into the cylinder cavity and thereby disappears. If the dosing is performed as desired while the piston rings are disposed opposite the lubricating units, it is not so critical, but if the dosing occurs outside this period, there are no benefits from a part of the dosed oil.
The two above mentioned methods may also be said to concern a system where lubrication is established by piston distribution of the lubricating oil.
A third method for cylinder lubrication uses systems that feed the lubricating oil directly into the cylinder, directly onto the cylinder wall and before passage of the piston.
In these systems an injector is used which either supplies the lubricating oil in atomised form or in the shape of one or more compact jets. For supplying the lubricating oil to the injector, either a traditionally mechanically driven lubricating apparatus or a hydraulic apparatus is used.
The advantage of this method is that the lubricating oil is already largely distributed on the cylinder wall before passage of the piston. According to this method, the oil is distributed at the top of the cylinder before arrival of the piston, and it is expected that the piston during the expansion stroke carries lubricating oil down into the cylinder. Examples of this technique are known from e.g. WO 0028194, EP 1 350 929 or DK 176 129.
In EP 1 350 929 is described a method where lubricating oil jets—where atomisation of the lubricating oil is avoided to the greatest extent—can be delivered to the cylinder face by injection before, during and/or after passage of the piston. This means that the total amount of lubricating oil is injected onto the cylinder face in at least two parts as indicated in the introduction.
Since the cylinder wall is supplied with oil before passage of the piston, the timing is not so important by this third method as by the two first mentioned systems where the oil is to be supplied exactly in the course of the very short interval when the piston rings are situated opposite the lubricating unit.
Examination has shown that cylinder lubrication according to WO 0028194, so-called SIP lubrication, provides the highest oil film thickness in the cylinder where the wear is the greatest, corresponding to the piston being in top position and in the area of the uppermost piston ring. In contrast to this it has appeared that conventional lubrication or high-speed lubrication provides a thicker oil film on the rest of the travel surface.
The pressure existing by SIP lubrication is required in the lubricating oil lines between pumps and nozzles in order to ensure that the intended atomisation is considerably higher than the pressure by the conventional lubricating methods which operate with pressures of a few bars. SIP valves operate at a preset pressure of 35-40 bars.
The supplying of lubricating oil has furthermore the purpose of neutralising the acid action on the cylinder wall. The acid action arises by combustion of sulphur-containing fuels and they are best counteracted by supplying the lubricating oil directly at the top of the cylinder. Measurements shown that the SIP lubrication provides the least wear. In practice it appears that corrosive wear is the most critical factor for the service life of a cylinder.
A drawback of conventional lubrication or high-speed lubrication, which both are systems that mainly use the piston for distributing the lubricating oil, is that a certain excessive lubrication is needed in order to ensure sufficient lubricating oil for the top of the cylinder. In particular lubrication on the piston requires an increase of the amount of lubricating oil in relation to the sulphur content of the fuel in order to achieve satisfactory cylinder conditions.
Correspondingly, for lubrication with systems where the lubricating oil is fed directly onto the cylinder wall it may be a disadvantage that an insufficient amount of oil is provided at the bottom of the cylinder when applying an amount of lubricating oil sufficient for obviating corrosive wear. This is due to the fact that the piston rings, besides the above mentioned distributing function, also produce a certain scraping action. Measurements show that SIP lubrication produce less scraping down of lubricating oil than lubrication with piston-distributed lubricating oil.
Another difference of lubricating with systems where the lubricating oil is supplied directly to the cylinder wall and piston-distributed lubrication is a consequence of different amounts of lubricating oil being provided down in the cylinder. The scavenge drain oil is thus measurably less by SIP lubrication (according to WO 0028194) than by systems with piston-distributed lubrication where it is only the piston that distributes the lubricating oil. This means that one of the parameters used for assessing the cylinder condition—namely measurement of Fe-content in the scavenge drain oil—cannot be used directly by comparing the cylinder condition since the same Fe-content will give rise to a concentration that varies depending on the lubrication method.
The scavenge air apertures in longitudinally scavenged two-stroke diesel engines are disposed in such a way that during scavenging, a rotational movement of the gas mixture is started simultaneously with the gas being displaced upwards in the cylinder, leaving it through the exhaust valve at the top of the cylinder. The gas in the cylinder thus follows a helical path or whirl on its way from the scavenge air apertures to the exhaust valve. Due to the centrifugal force, a sufficiently small oil particle located in this whirl will be forced out against the cylinder wall, eventually becoming deposited on the wall. This effect is utilised by introducing the oil portions into the cylinder as a mist of oil particles of suitable size, atomised through nozzles. By adjusting the size of the nozzles, the ejection speed and pressure of the oil before the nozzles, it is possible to control the average size of the oil droplets in the oil mist. If an oil particle or oil droplet is too small, it will “float” too long in the gas stream, eventually being moved away by the scavenging air without hitting the cylinder wall. If it is too large, it will continue too far in its initial path due to its inertia and not reach the cylinder wall, which is due to it being overtaken by the piston and positioned at the top of the piston.
The orientation of the nozzles relative to the flow in the cylinder may be arranged such that the interaction between individual droplets and the gas stream in the cylinder ensures that the oil droplets hit the cylinder wall over an area largely corresponding to the circumferential distance between two lubricating points. In this way, the oil is even distributed more or less uniformly across the cylinder surface before the passage of the piston rings. Besides, the nozzle may be adjusted such that the oil hits the cylinder wall higher up than the nozzles. Thus, before being introduced into the cylinder, the oil will not only be better distributed across the cylinder surface, but will also be distributed on the cylinder surface closer to the top the cylinder where the need for lubrication is the greatest. Both of these facts will result in improved utilisation of the oil with assumed improvement of the relation between the service life of the cylinder and the oil consumption.
The supply of oil to the cylinder surface is to be effected in measured portions which is almost the case with the two previously mentioned traditional systems. The supply means can be traditional lubricating systems, but other supply means with corresponding properties may also be envisaged.
In order to ensure that the pressure in the cylinder does not go backwards in the oil line, a check valve is arranged in a normal way at the end of the lubrication line immediately before the lining of the inner cylinder face. The check valve allows the oil to pass from the oil line to the cylinder lining, but does not let gas pass in the opposite direction. These check valves usually have a modest opening pressure (a few bars).
Characteristics of the three above mentioned methods for lubricating cylinders are:                Lubrication timing—when is the lubricating oil supplied in the engine cycle?        Supply amount—how is the relative injected amount adjusted?        Pump characteristic—how and how fast is the lubricating oil supplied?        
It is relevant to look for methods of minimising the lubricating oil consumption by providing an improvement of the cylinder lubrication of large diesel engines, such as marine engines.